Cancer of the breast is a major cause of death among the American female population. Effective treatment of this disease is most readily accomplished following early detection of malignant tumors. Major efforts are presently underway to provide mass screening of the population for symptoms of breast tumors. Such screening efforts will require sophisticated, automated equipment to reliably accomplish the detection process.
The x-ray absorption density resolution of present photographic x-ray methods is insufficient to provide reliable early detection of malignant tumors. Research has indicated that the probability of metastasis increases sharply for breast tumors over 1 cm in size. Tumors of this size rarely produce sufficient contrast in a mammogram to be detectable. To produce detectable contrast in photographic mammogram 2-3 cm dimensions are required. Calcium deposits used for inferential detection of tumors in conventional mammography also appear to be associated with tumors of large size. For these reasons, photographic mammography has been relatively ineffective in the detection of this condition.
Most mammographic apparatus in use today in clinics and hospitals require breast compression techniques which are uncomfortable at best and in many cases painful to the patient. In addition, x-rays constitute ionizing radiation which injects a further risk factor into the use of mammographic techniques as most universally employed.
Ultrasound has also been suggested as in U.S. Pat. No. 4,075,883, which requires that the breast be immersed in a fluid-filled scanning chamber, U.S. Pat. No. 3,973,126 also requires that the breast be immersed in a fluid-filled chamber for an x-ray scanning technique.
In recent times, the use of light and more specifically laser light to non-invasively peer inside the body to reveal the interior structure has been investigated. This technique is called optical imaging. Optical imaging and spectroscopy are key components of optical tomography. Rapid progress over the past decade have brought optical tomography to the brink of clinical usefulness. Optical wavelength photons do not penetrate in vivo tissue in a straight line as do x-ray photons. This phenomena causes the light photons to scatter inside the tissue before the photons emerge out of the scanned sample.
Because x-ray photons propagation is essentially straight-line, relatively straight forward techniques based on the Radon transform have been devised to produce computed tomography images through use of computer algorithms. Multiple measurements are made through 360.degree. around the scanned object. These measurements, known as projections, are used to back-project the data to create an image representative of the interior of the scanned object.
Another aspect of image reconstruction algorithm development relates to certain assumptions concerning the optical path through a scanned object starting at the point at which the radiation beam of photons initially enters the scanned object and the point on the scanned object at which the photons finally exit. The basis for the required assumptions is that there is no direct way of visualizing the actual optical path through a scanned object, in particular in vivo breast tissue, and measurements made through use of experimental models or phantoms provide empirical information of probable optical paths. Through knowledge of the physical relationship of the radiation beam, the scanned object's perimeter, and the respective sensor or sensors, reconstruction algorithm developed is possible.
In optical tomography, the process of acquiring the data that will ultimately be used for image reconstruction is the first important step. Light photon propagation is not straight-line and techniques to produce cross-sectional images are mathematically intensive. To achieve adequate spatial resolution, multiple sensors are employed to measure photon flux density at small patches on the surface of the scanned object. The volume of an average female breast results in the requirement that data must be acquired from a large number of patches. The photon beam attenuation induced by the breast tissue reduces the available photon flux to an extremely low level and requires sophisticated techniques to capture the low level signals.
Methods to acquire the scanning data are engineering issues that acquire low level signals, simultaneous data acquisition from a large number of sensors surrounding the breast, multiple levels of sensors surrounding the breast to allow rapid data acquisition, and both rotation and translation control of sensors and laser beam.
The present invention provides efficient methods for acquisition of low level photon flux signals from multiple locations around the scanned breast.